Significantly higher-density recording has been demanded in connection with the development of magnetic recording mediums in recent years. Attempts to smooth the surface of the magnetic layer have been made as a possible means for attaining enhanced high-density recording.
However, when the surface of the magnetic layer is smoothed, a coefficient of friction of the magnetic layer with each part it comes into contact with in a recording and reproducing apparatus is increased during the running of magnetic recording mediums. As a result, there is a possibility that running troubles in use are caused, that the magnetic layer is damaged to thereby increase drop-out, and that the magnetic layer is peeled off its non-magnetic support.
Additionally, in magnetic recording mediums having a magnetic layer composed of a ferromagnetic metallic thin film, that is, metallic thin film type magnetic recording mediums which are designed for use as high-density recording mediums and extensively studied to develop and bring them to practical use towards that objective, the surface of the magnetic layer is very smooth so that the above-described problems caused by increased coefficients of friction are even more pronounced.
To solve the above problems, prior attempts have been made to incorporate lubricants such as fatty acids, fatty acid esters, hydrocarbons or silicone compounds into the magnetic layer or the surface thereof. However, magnetic layers have insufficient durability when incorporating these conventional lubricants.
With the popularization of VTR, personal computers and word processors, floppy disc drive type apparatuses have become more popular in recent years and magnetic recording mediums have been used over a wide range of environments such as under low-temperature conditions and under high-temperature and high-humidity conditions.
Accordingly, magnetic recording mediums are demanded which have stable running durability under a wide range of environmental conditions.
The application of fluorinated oils such as perfluoropolyethers to metallic thin film type magnetic mediums has been examined. For example, U.S. Pat. Nos. 4,267,238 and 4,268,556, West German Patent 3,000,583 and JP-B-60-10368 (the term "JP-B" as used herein means an "examined Japanese patent publication") disclose perfluoropolyethers whose terminals are modified by addition of polar groups to enhance fixation to the surface of the magnetic layer.
Further, attempts have been made to provide good lubricity as well as fixation to the surface of the magnetic layer by using a combination of a perfluoropolyether having a polar group with a perfluoropolyether having no polar group as described in JP-A-61-113126 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 4,682,378.
However, the above-described coefficient of friction problem could not be adequately solved by using these conventional perfluoropolyether lubricants.
The present invention is intended to solve such problems associated with the prior art as mentioned above.